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Based on kernel version 3.15.4. Page generated on 2014-07-07 09:04 EST.

1	Using RCU to Protect Read-Mostly Arrays
2	
3	
4	Although RCU is more commonly used to protect linked lists, it can
5	also be used to protect arrays.  Three situations are as follows:
6	
7	1.  Hash Tables
8	
9	2.  Static Arrays
10	
11	3.  Resizeable Arrays
12	
13	Each of these situations are discussed below.
14	
15	
16	Situation 1: Hash Tables
17	
18	Hash tables are often implemented as an array, where each array entry
19	has a linked-list hash chain.  Each hash chain can be protected by RCU
20	as described in the listRCU.txt document.  This approach also applies
21	to other array-of-list situations, such as radix trees.
22	
23	
24	Situation 2: Static Arrays
25	
26	Static arrays, where the data (rather than a pointer to the data) is
27	located in each array element, and where the array is never resized,
28	have not been used with RCU.  Rik van Riel recommends using seqlock in
29	this situation, which would also have minimal read-side overhead as long
30	as updates are rare.
31	
32	Quick Quiz:  Why is it so important that updates be rare when
33		     using seqlock?
34	
35	
36	Situation 3: Resizeable Arrays
37	
38	Use of RCU for resizeable arrays is demonstrated by the grow_ary()
39	function used by the System V IPC code.  The array is used to map from
40	semaphore, message-queue, and shared-memory IDs to the data structure
41	that represents the corresponding IPC construct.  The grow_ary()
42	function does not acquire any locks; instead its caller must hold the
43	ids->sem semaphore.
44	
45	The grow_ary() function, shown below, does some limit checks, allocates a
46	new ipc_id_ary, copies the old to the new portion of the new, initializes
47	the remainder of the new, updates the ids->entries pointer to point to
48	the new array, and invokes ipc_rcu_putref() to free up the old array.
49	Note that rcu_assign_pointer() is used to update the ids->entries pointer,
50	which includes any memory barriers required on whatever architecture
51	you are running on.
52	
53		static int grow_ary(struct ipc_ids* ids, int newsize)
54		{
55			struct ipc_id_ary* new;
56			struct ipc_id_ary* old;
57			int i;
58			int size = ids->entries->size;
59	
60			if(newsize > IPCMNI)
61				newsize = IPCMNI;
62			if(newsize <= size)
63				return newsize;
64	
65			new = ipc_rcu_alloc(sizeof(struct kern_ipc_perm *)*newsize +
66					    sizeof(struct ipc_id_ary));
67			if(new == NULL)
68				return size;
69			new->size = newsize;
70			memcpy(new->p, ids->entries->p,
71			       sizeof(struct kern_ipc_perm *)*size +
72			       sizeof(struct ipc_id_ary));
73			for(i=size;i<newsize;i++) {
74				new->p[i] = NULL;
75			}
76			old = ids->entries;
77	
78			/*
79			 * Use rcu_assign_pointer() to make sure the memcpyed
80			 * contents of the new array are visible before the new
81			 * array becomes visible.
82			 */
83			rcu_assign_pointer(ids->entries, new);
84	
85			ipc_rcu_putref(old);
86			return newsize;
87		}
88	
89	The ipc_rcu_putref() function decrements the array's reference count
90	and then, if the reference count has dropped to zero, uses call_rcu()
91	to free the array after a grace period has elapsed.
92	
93	The array is traversed by the ipc_lock() function.  This function
94	indexes into the array under the protection of rcu_read_lock(),
95	using rcu_dereference() to pick up the pointer to the array so
96	that it may later safely be dereferenced -- memory barriers are
97	required on the Alpha CPU.  Since the size of the array is stored
98	with the array itself, there can be no array-size mismatches, so
99	a simple check suffices.  The pointer to the structure corresponding
100	to the desired IPC object is placed in "out", with NULL indicating
101	a non-existent entry.  After acquiring "out->lock", the "out->deleted"
102	flag indicates whether the IPC object is in the process of being
103	deleted, and, if not, the pointer is returned.
104	
105		struct kern_ipc_perm* ipc_lock(struct ipc_ids* ids, int id)
106		{
107			struct kern_ipc_perm* out;
108			int lid = id % SEQ_MULTIPLIER;
109			struct ipc_id_ary* entries;
110	
111			rcu_read_lock();
112			entries = rcu_dereference(ids->entries);
113			if(lid >= entries->size) {
114				rcu_read_unlock();
115				return NULL;
116			}
117			out = entries->p[lid];
118			if(out == NULL) {
119				rcu_read_unlock();
120				return NULL;
121			}
122			spin_lock(&out->lock);
123	
124			/* ipc_rmid() may have already freed the ID while ipc_lock
125			 * was spinning: here verify that the structure is still valid
126			 */
127			if (out->deleted) {
128				spin_unlock(&out->lock);
129				rcu_read_unlock();
130				return NULL;
131			}
132			return out;
133		}
134	
135	
136	Answer to Quick Quiz:
137	
138		The reason that it is important that updates be rare when
139		using seqlock is that frequent updates can livelock readers.
140		One way to avoid this problem is to assign a seqlock for
141		each array entry rather than to the entire array.
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